Uma abordagem sistemática em espécies de Hypostomus Lacépède, 1803 (Siluriformes, Loricariidae) da Bacia do rio São Francisco através da análise citogenética básica e molecular

Abstract

Loricariidae is the richest family among the Siluriformes, with 116 genera and 1,071 valid species distributed distributed in six subfamilies: Delturinae, Hypoptopomatinae, Hypostominae, Lithogeninae, Loricariinae, and Rhinelepinae. Almost half of all species are assigned to the subfamily Hypostominae, with Hypostomus being the most speciose genus, comprising 154 recognized species, of which only 30 have cytogenetic data. A phylogeny based on molecular characters with 108 species was recently presented, confirming the monophyly of the genus and presenting four main lineages called supergroups: Hypostomus cochliodon, Hypostomus hemiurus, Hypostomus auroguttatus, and Hypostomus plecostomus. The São Francisco River is the largest exclusively Brazilian river, with 14 described species of Hypostomus, none of which have been studied cytogenetically. Given this scenario of scarce data, and the recent phylogenetic proposal presented, basic and molecular cytogenetic analyses were performed on six Hypostomus species from the São Francisco River basin (H. alatus, H. francisci, H. freirei, H. guajupia, H. lima, Hypostomus sp.) in order to verify evolutionary trends in the supergroups. Hypostomus alatus presented 2n=72 chromosomes (10m + 28sm + 28st + 6a), simple NORs (Ag- and 18S rDNA), reduced amount of interstitial heterochromatin, and multiple 5S rDNA sites (4 pairs). Hypostomus franscici presented 2n=72 chromosomes (10m + 30 sm+ 26st + 6a), simple NORs (Ag- and 18S rDNA), reduced amount of interstitial heterochromatin, and multiple 5S rDNA sites (5 pairs). Hypostomus freirei presented 2n=72 chromosomes (12m + 20sm + 28st + 12a for females; 12m + 21sm + 27st + 12a for males), evidencing a heteromorphic XX/XY sex chromosome system, simple NORs (Ag- and 18S rDNA), reduced amount of terminal heterochromatin, and multiple 5S rDNA sites (4 pairs). Hypostomus guajupia presented 2n=70 chromosomes (12m + 30sm + 16st + 12a), single NORs (Ag- and 18S rDNA), interstitial and centromeric heterochromatin, and multiple 5S rDNA sites (5 pairs). Hypostomus lima presented 2n=76 chromosomes (6m + 16sm + 24st + 30a), single NORs (Ag- and 18S rDNA), reduced amount of interstitial heterochromatin, and multiple 5S rDNA sites (2 pairs). Hypostomus sp. presented 2n=68 chromosomes (14m + 28sm + 18st + 8a), multiple NORs (Ag- and 18S rDNA), reduced amount of heterochromatin, and multiple 5S rDNA sites (3 pairs). The data allowed us to identify some trends in some supergroups and subgroups: 2n was effective for the supergroups H. cochliodon, H. plecostomus, and the H. asperatus group (included in the supergroup H. auroguttatus). Although there appears to be no general pattern in the location of rDNA sites (5S and 18S) within each supergroup, with species having single and multiple sites for both markers (with the exception of the H. asperatus group), the presence of a metacentric/submetacentric pair carrying a 5S rDNA site in a centromeric/pericentromeric position on the short arm is a characteristic observed in Hypostomus species. Regarding the distribution of heterochromatin, the studies show heterogeneity in its distribution and composition, and it likely represents an important marker for taxonomic studies, since they reveal species-specific distribution and composition within the genus. The use of cytogenetics in Hypostomus has proven to be an efficient tool both in species diagnosis and in proposing groupings, and should be considered in future phylogenetic studies.